The present invention relates to a novel method for preparation of olefin oligomers and to be precise, it relates to a method for preparation of olefin oligomers which can prevent halogen atoms from being present in the polymerization product and which will provide olefin oligomers having a low viscosity and a high viscosity index.
As a method for polymerization of olefins, the cationic polymerization method employing Lewis acids such as aluminum chloride has been known for a long time. According to this method, however, the degree of polymerization becomes so high under the ordinary reaction conditions that there can be obtained only high-viscosity polyolefin oils. Consequently, a polyolefin oil prepared by the conventional cationic polymerization is in fact disqualified for use as a gas turbine oil, a hydraulic fluid for aircraft, an insulating oil, a cosmetic base and the like. It is admittedly possible to obtain a polyolefin oil having a low viscosity even by the use of aluminum chloride if the reaction temperature is sufficiently high. For instance, a polyolefin oil having a viscosity of 30 centistokes or thereabouts can be obtained by polymerizing octene-1 with aluminum chloride at about 180.degree.C, but in this case, the product polyolefin oil is defective in that not only is the viscosity index thereof as low as 100 - 105 (VIE), but also the halogen of the catalyst is present therein to the extent of several thousand ppm.
The presence of halogen in the polymerization product has a bad influence of grave importance on the process of after-treatment of the polymerization product. For instance, at the time of distilling the unreacted olefin monomer and/or olefin dimer from the polymerization product, there takes place a thermal cracking of a part of the halogen in the polyolefin oil thereby to generate hydrogen chloride, so that the distillation apparatus is corroded thereby. Furthermore, at the time of the hydrogenation of the double bonds remaining in the polymerization product, carried out for the purpose of improving the heat stability as well as the oxidation stability of the polymerization products, there occurs trouble that the Raney nickel used as catalyst for said hydrogenation is rendered inactive by the halogen.
Under these circumstances, there has been proposed the method employing an alkaline aqueous solution or an aqueous ammonia, the method employing an absorbent, etc., for the purpose of decomposing and removing the polymerizaton catalyst prior to subjecting said polymerization product to distillation. All of these methods, however, are defective in that the polyolefin oil obtained thereby contains therein residual halogen to the extent of several thousand or more ppm, so that the cationic polymerizaton method at high temperature employing aluminum chloride has practically been out of use.
There has been proposed in Japanese Pat. Publication No. 3804/1969 a method of effecting polymerization of olefins in the presence of a liquid catalyst prepared by dissolving excess aluminum halide in a complex consisting of aluminum halide and ethyl acetate at the molar ratio of 1:1. However, inasmuch as said complex does not act as a catalyst, the effect of this method is no more than cationic polymerization employing the aluminum halide added in excess. Therefore, even by this method employing a complex, there can be obtained only a polyolefin oil having a high viscosity under the ordinary reaction conditions, like in the case of the foregoing cationic polymerization method. If the reaction temperature is elevated in order to obtain a polyolefin oil having a low viscosity, the viscosity index of the product polyolefin oil will be reduced, causing the same trouble as in the case of the above discussed aluminum chloride catalyst, namely, halogen atoms are present in the product polyolefin oil.
The present invention proposes a novel polymerization method and its object is to prepare an olefin oligomer having a low viscosity and a high viscosity index and which is practically free from halogen atoms therein.